What are mountain breezes and valley breezes, and why do mountain breezes and valley breezes alternate daily between mountain areas and valley areas depending on the time of day?
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A local wind system of a mountain valley that blows downhill (mountain breeze) at night and uphill (valley breeze) during the day is called mountain and valley breeze.
During the day, the thin air above the high mountainsides warms quickly. The warm air rises and creates local low pressure along the slopes. Air from the lower valleys moves in to replace it, creating an upslope breeze that becomes strongest around noon. This is the valley breeze.
At night, the high mountain slopes cool very quickly. This cold, dense air forms a local high-pressure area. The pressure gradient drives a gentle breeze down the slope into the valley that is strongest just before sunrise. This is the mountain breeze.
The most basic differences are the time of day, and direction of the breezes. During day time, the heat coming off the mountain will heat air higher up in the atmosphere at a faster rate then the ground (in the valley) can. Warm air expands, which creates a small low pressure system at or near the mountain top. In turn, this draws in air from the valley, thus creating a breeze that blows up from the valley to the top of the mountain. The updraft pattern is called valley breeze.
At night, the sheer mountain slopes cool down air faster than the existing air in the atmopshere. This creates a localized high pressure system, as the air becomes more dense and packed in. That high pressure makes winds blow down the mountain into the valley. This downdraft pattern is called mountain breeze.
To summarize-- Valley Breeze: daytime winds blowing from the valley up to the mountains. Mountain Breeze: nighttime winds that go from the mountains down to the valley.
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